receive a very high solar radiation of ca. 195 kCal cm
-2
y
−
1
[18]
(
Fig. 3
). In a similar environment, the yearly duration of sunshine in
Lhasa was reported to be 3021 h, while those in Chengdu and Shanghai
were 1186 and 1932 h, respectively
[18]
.
Australia has the highest solar radiation per square meter of all
continents and is estimated to have the best solar energy resource in the
world. On the Australian continent, a relatively high daily solar irra-
diance of 4
–
6 kW h m
−
2
has been reported
[20]
. Comparable levels of
solar radiation were also observed in the desert areas of northern and
southern Africa, the southwestern United States, adjacent areas of
Mexico, and regions of the Paci
fi
c coast o
ff
South America
[13]
.
The whole solar energy concept is regarded as the harvesting and
utilization of light and/or heat energy generated by the Sun and tech-
nologies (passive and active) involved in achieving such goals
[21]
. A
classi
fi
cation of present solar energy technologies is shown in
Fig. 4
. By
de
fi
nition, passive technology involves the accumulation of solar en-
ergy without transforming thermal or light energy into any other form
(for power generation, for instance)
[22]
. Solar energy collection, sto-
rage, and distribution in the form of heat for the heating of homes
(especially during the winter season) exempli
fi
es a form of passive solar
technology. On the other hand, active solar system collects solar ra-
diation and uses mechanical and electrical equipment (e.g., pumps or
fans) for the conversion of solar energy to heat and electric power. The
most well known application of this system is the solar water heater
system.
In general, active solar energy technology can be further grouped
into two categories: (i) photovoltaic technology and (ii) solar thermal
technology
[23]
. In recent years, photovoltaic technology involving the
use of semiconductors to convert sunlight directly into electrical energy
has become a highly desirable option
[24]
. The intense research e
ff
orts
of energy scientists with regard to solar options have helped to yield an
improved e
ffi
ciency of photovoltaic technology; in case of hybrid per-
ovskite solar cells ((CH
3
NH
3
)PbI
3
), a promising achievement (e.g., an
e
ffi
ciency increase of ca. 18%) has been reported
[25,26]
. Currently,
photovoltaic technology involving wafer-based cells (traditional crys-
talline silicon or gallium arsenide), commercial thin-
fi
lm cells (cad-
mium telluride, amorphous silicon, copper indium gallium diselenide),
and new thin-
fi
lm technologies (perovskites, organic materials,
quantum dots) are progressing with the advent of intense R & D e
ff
orts
[27]
.
In solar thermal technology, solar energy is harnessed into thermal
energy for domestic and/or commercial applications such as drying,
heating, cooling, cooking, etc.
[28,29]
. However, on the industrial
scale, concentrated solar thermal (CST) technologies are being used to
ful
fi
ll such heating requirements while concentrated solar power (CSP)
technologies are being employed to generate electricity. The latter in-
volves the use of high-magni
fi
cation mirrors to concentrate solar energy
prior to converting it into heat energy to power a steam turbine
[30]
.
Four types of CSP technologies are currently available on the open
market: (i) parabolic troughs (these concentrate sunlight onto a receiver
tube containing a working liquid); (ii) Fresnel mirrors (use multiple
fl
at
mirrors to concentrate solar sunlight onto a receiver tube); (iii) power
towers (an array of thousands of sun-tracking re
fl
ecting mirrors posi-
tioned in a
fi
eld to concentrate solar radiation to a single point), and
(iv) solar dish collectors (concentrate power by focusing ST energy onto
a single point situated above a re
fl
ector dish)
[31]
.
2.1. Current global status for solar energy
The availability of most renewable energy sources (i.e., wind, solar,
tidal wave, hydro, etc.) tends to vary widely throughout the course of a
day, season, year, even from one geographical location to another
[32]
.
A comparison of the global power capacities between di
ff
erent
Fig. 3.
Annual mean distribution of direct solar radiation in China
(unit: kWh/m
2
)
[19]
.
Fig. 4.
Classi
fi
cation of the present solar energy technologies.
E. Kabir et al.
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